"The rules fall down when, through no special effort, one character outshines everyone else combined. There’s also a problem when entire classes don’t have much to contribute. The system works if the average gamer can put together a character and have a good time. Thus, overall the classes should be balanced by giving them unique and interesting things to do that allow each one to stand out in its own way."

Dungeons & Dragons Roleplaying Game Official Home Page - Article (RPG Design Philosophy)